In the field related to the recent electronic equipment, a pattern for wiring on a plated printed circuit used therein becomes finer and finer. As for a metallic thin film layer used for forming various electrode patterns, growing progress is also made in application of the metallic thin film layer with an extremely thin film thickness. For instance, when attaining formation of a fine patter for wiring and a thin film by means of a screen printing method, increasing attempt is made in application of a dispersion of metallic fine particles with an extremely small particle diameter to the drawing of an ultra fine pattern or the formation of a thin-film coating layer with an extremely thin film-thickness. In the present, a dispersion of gold or silver fine particles, which is applicable to the above-described use, has been already commercialized.
Among them, as for a process for forming an ultra fine wiring pattern with the use of metallic nano-particles, for instance, a process scheme has been already established for the case where gold nano-particles or silver nano-particles are used. Specifically, formation of wiring with a wiring width and space between wires of 5 to 50 μm and a volume resistivity of 1×10−5 Ω·cm can be made with a sintered product type of wiring layer obtained by drawing the extremely fine wiring pattern with the use of the dispersion for ultra fine printing containing the gold nano-particles or the silver nano-particles, and then sintering metallic nano-particles with each other. However, in the case where the gold nano-particles are employed, as gold itself of a material is expensive, a unit cost of the production of the dispersion for the ultra fine printing also becomes expensive, which is an economic bottleneck in wide spreading as a general purpose product. On the other hand, in the case where the silver nano-particles are used in place, the unit cost for the production of said dispersion can be considerably reduced, but as-the wiring width and the space between wires become narrow, such another serious problem as the breaking of a wire resulted from the event of electro-migration comes up to the surface.
For the purpose of avoiding the breaking of the wire caused by the electromigration phenomenon, the application of a copper-based wiring is effective, and, for instance, the use of the copper-based material for the wiring pattern on a semiconductor device is widely studied as much higher integration is intended. Specifically, copper shows high electroconductivity similar to gold and silver, and has good ductility and malleability, but far lower electromigration is observed for copper in comparison with silver. Accordingly, when a fine wiring gives rise to the increased current density, use of the copper-based wiring can avoid the breaking of the wire caused by the electromigration phenomenon.
Hence, as for a plated printed circuit, when producing a fine wiring pattern with a sintered metallic wiring layer obtained by sintering metallic fine particles, for instance, metallic nano-particles with each other, it is desired to use copper, which shows little electromigration, as well. There are entertained great hopes for successful use of copper with the aim at reduction in cost for production of a plated printed circuit having a fine wiring pattern for wider spectra of purpose, as copper itself has far cheaper unit cost of the material in comparison with gold and silver.
Gold and silver, which are included in examples of noble metals, originally have such a feature that they may be hardly oxidized in comparison, so that when preparing the dispersion of fine particles, it is easy to keep the contained fine particles into the state of forming no oxide film on the surface. On the other hand, copper has such a feature that it may be easily oxidizable by comparison, so that when the dispersion of the fine particles has been prepared, the contained fine particles fall, in a short time, into such a state that they have an oxide film formed on their surfaces. Particularly, in the case of a copper nano-particle with a finer particle diameter, it has a relatively increased surface area, and has an oxide film with an increased thickness formed on its surface, and thus there are by no means few cases that most part of particle diameter of nanometric size is converted into the surface oxide film layer made of copper oxides.